1. Field of the Invention
The present invention relates to cathode electrodes within electrolytic capacitors and, more particularly, to the use of a titanium or similar metal or metal alloy electrode as a cathode electrode within an electrolytic capacitor for use in pulse discharge applications and to an electrolytic capacitor including the cathode electrode of the present invention.
2. Related Art
Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density since it is desirable to minimize the overall size of the implanted device. This is particularly true of an Implantable Cardioverter Defibrillator (ICD), also referred to as an implantable defibrillator, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
Implantable Cardioverter Defibrillators, such as those disclosed in U.S. Pat. No. 5,131,388, incorporated herein by reference, typically use two electrolytic capacitors in series to achieve the desired high voltage for shock delivery. For example, an implantable cardioverter defibrillator may utilize two 350 to 400 volt electrolytic capacitors in series to achieve a voltage of 700 to 800 volts.
Electrolytic capacitors are used in ICDs because they have the most nearly ideal properties in terms of size, reliability and ability to withstand relatively high voltage. Conventionally, such electrolytic capacitors typically consist of a cathode electrode, an electrically conductive electrolyte and a porous anode with a dielectric oxide film formed thereon. While aluminum is the preferred metal for the anode plates, other metals such as tantalum, magnesium, titanium, niobium, zirconium and zinc may be used. A typical electrolyte may be a mixture of a weak acid and a salt of a weak acid, preferably a salt of the weak acid employed, in a polyhydroxy alcohol solvent. The electrolytic or ion-producing component of the electrolyte is the salt that is dissolved in the solvent. The entire laminate is rolled up into the form of a substantially cylindrical body, or wound roll, that is held together with adhesive tape and is encased, with the aid of suitable insulation, in an aluminum tube or canister. Connections to the anode and the cathode are made via tabs. Alternative flat constructions for aluminum electrolytic capacitors are also known, comprising a planar, layered, stack structure of electrode materials with separators interposed therebetween, such as those disclosed in the above-mentioned U.S. Pat. No. 5,131,388.
The need for high voltage, high energy density capacitors is most pronounced when employed in implantable cardiac defibrillators (ICDs). In ICDs, as in other applications where space is a critical design element, it is desirable to use capacitors with the greatest possible capacitance per unit volume. Since the capacitance of an electrolytic capacitor is provided by the anodes, a clear strategy for increasing the energy density in the capacitor is to minimize the volume taken up by paper and cathode and maximize the number of anodes. A multiple anode flat, stacked capacitor configuration requires fewer cathodes and paper spacers than a single anode configuration and thus reduces the size of the device. A multiple anode stack consists of a number of units consisting of a cathode, a paper spacer, two or more anodes, a paper spacer and a cathode, with neighboring units sharing the cathode between them. In order to achieve higher energy densities, it has been necessary to stack three, four and five anodes per layer. However, due to the higher capacitance values achieved with multiple anodes, traditional chemically etched aluminum cathodes provide insufficient capacitance coverage at the desired thickness of 30 microns or less.
It is well understood that, in order to achieve high total capacitance and maximum anode gain realization, the cathode capacitance must be nearly two orders of magnitude higher than the anode stack capacitance that is opposes. When the cathode capacitance is much larger than the anode stack capacitance, the cathode electrode maintains a negative potential. However, if the cathode capacitance is not much larger than the anode capacitance, the cathode electrode can develop a positive potential. If the cathode develops a positive potential, several undesirable effects can occur including oxide buildup on the cathode which reduces the capacitor performance, electrolysis that consumes electrolyte and deteriorates the performance of the capacitor with usage, and production of gaseous electrolysis byproducts that can cause swelling of the capacitor.
Conventional chemically etched aluminum cathodes have been composed of 3003-alloy hard aluminum film of 20 to 70 micron thickness. The aluminum is typically etched in hydrochloric acid (HCl) with or without additional additives. Typical capacitances from such cathodes are given in FIG. 1. A conventional 30 micron aluminum cathode does not provide an adequate anode/cathode capacitance ratio for full 120 Hz capacitance realization in a multiple anode flat, stacked capacitor configuration. In order to realize the anode foil capacitance, a high capacitance cathode foil is needed.
Conventionally, high capacitance cathodes are obtained by using a thin surface-area-enhanced foil with minimal oxide present. Chemical and electrochemical etch processes and deposition processes have been used to increase surface area. Additionally, it is known to coat high surface area materials on to metal foil substrates, such as titanium nitride on aluminum, metal oxides on titanium, or conductive polymers on a variety of metals including aluminum and titanium. For example, the assignee of the present invention has previously used a titanium nitride (TiNx) coated aluminum cathode, purchased from Becromal of America, Inc. of Clinton, Tenn. under the product name BECROMAL Kappa 30B black cathode. A cathode capacitance of 600 to 800 μF/cm2 was obtainable in a 30 micron thick BECROMAL Kappa 30B black cathode foil. However, Kappa 30B has been discontinued in the United States.
Known high capacitance cathode technologies present significant problems. Etched aluminum cathodes are technologically limited and can not support the high capacitance required when more than two anodes are placed adjacent to each other in a capacitor stack. Coated cathodes are difficult to obtain commercially and reliably. The titanium nitride coating process is expensive. Metal oxides are difficult to coat in high purity without chloride contamination. Conductive polymers have exhibited stability issues under certain loads and uses. Therefore, there is a need for a cathode that provides suitable coverage to allow for a multi-anode configuration with maximum energy output.